1. Field of the Invention
The present invention relates to an optical fiber applicable to a transmission line in optical communications, and an optical transmission system including this optical fiber.
2. Related Background Art
Conventionally, as a transmission line in optical communications, standard single-mode optical fibers having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band (1280 nm to 1320 nm) have mainly been utilized. The transmission loss resulting from the main material (silica) of such an optical fiber has been known to become the lowest in a 1.55-xcexcm wavelength band (1530 nm to 1565 nm). In addition, optical fiber amplifiers using an Er-doped optical fiber can amplify light in the 1.55-xcexcm wavelength band at a high efficiency. For such a reason, dispersion-shifted optical fibers designed so as to have a zero-dispersion wavelength in the 1.55-xcexcm wavelength band are applied to transmission lines in wavelength division multiplexing (WDM) communications for transmitting a plurality of wavelengths of signal light. As for a light source for sending out signal light, device technologies for enabling light in the 1.3-xcexcm wavelength band and light in the 1.55-xcexcm wavelength band to be outputted have conventionally been established.
The inventors have studied the prior art mentioned above and, as a result, found problems as follows. Namely, in the case where light in the 1.3-xcexcm wavelength band is transmitted while a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-xcexcm wavelength band is used as an optical transmission line, the absolute value of dispersion becomes so large that WDM communications cannot be carried out in a wide band. Also, when signal light in the 1.55-xcexcm wavelength band is transmitted through such a dispersion-shifted optical fiber, the absolute value of dispersion becomes so small that four-wave mixing, which is one of nonlinear optical phenomena, is likely to occur. In the case where light in the 1.3-xcexcm wavelength band is transmitted while a standard single-mode optical fiber having a zero-dispersion wavelength in the 1.3-xcexcm wavelength band is used as an optical transmission line, on the other hand, the absolute value of dispersion becomes so small that four-wave mixing, which is one of nonlinear optical phenomena, is likely to occur. Also, when signal light in the 1.55-xcexcm wavelength band is transmitted through such a single-mode optical fiber, the absolute value of dispersion becomes so large that WDM communications cannot be carried out in a wide band.
For this matter, attempts have been made to develop optical fibers for suppressing the occurrence of dispersion over a wide wavelength band (see, for example, K. Okamoto et al., xe2x80x9cZero total in single-mode optical fibers over an extended spectral range,xe2x80x9d Radio Science, Volume 17, Number 1, pages 31-36, January-February 1982). For example, an optical fiber having a low dispersion value over a wide wavelength band has been proposed by yielding a large relative refractive index difference of 2.4% between its cladding region and core region and a small diameter of 3.5 xcexcm in the core region. However, it is difficult to make such an optical fiber having a very large relative refractive index difference between the cladding region and core region, and its transmission loss is large. In an optical fiber whose core region has a smaller diameter, on the other hand, the effective area becomes smaller, and nonlinear optical phenomena are likely to occur.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical fiber which enables efficient transmission of both of signal light in the 1.3-xcexcmwavelength band and signal light in the 1.55-xcexcm wavelength band, and an optical transmission system including the same.
The optical fiber according to the present invention is an optical fiber which enables efficient transmission of both of signal light in the 1.3-xcexcm wavelength band and signal light in the 1.55-xcexcm wavelength band, the optical fiber having only one zero-dispersion wavelength within a wavelength range of 1.20 xcexcm to 1.60 xcexcm and having a positive dispersion slope at the zero-dispersion wavelength. Here, this zero-dispersion wavelength lies within a wavelength range of 1.37 xcexcm to 1.50 xcexcm sandwiched between the 1.3-xcexcm wavelength band and the 1.55-xcexcm wavelength band. Also, the above-mentioned dispersion slope preferably has an absolute value of 0.10 ps/nm2/km or less at the above-mentioned zero-dispersion wavelength (preferably 0.06 ps/nm2/km or less at a wavelength of 1.55 xcexcm), and monotonously changes (e.g.,monotonously increases) at least in a wavelength range of 1.30 xcexcm to 1.55 xcexcm.
Thus, since this optical fiber has a zero-dispersion wavelength within the wavelength range of 1.37 xcexcm to 1.50 xcexcm including a wavelength of 1.38 xcexcm at which an increase in transmission loss caused by OH absorption is seen, dispersion occurs to a certain extent in the vicinity of the 1.3-xcexcm wavelength band and in the vicinity of the 1.55-xcexcm wavelength band. As a consequence, the optical fiber comprises a structure in which four-wave mixing is hard to occur even when the signal light in the 1.3-xcexcm wavelength band and the signal light in the 1.55-xcexcm wavelength band propagate therethrough.
In the case where a thulium-doped fiber amplifier having an amplification band in a 1.47-xcexcm wavelength band is utilized, the zero-dispersion wavelength is more preferably set within a wavelength range of 1.37 xcexcm to 1.43 xcexcm. It is because of the fact that the transmission band can further be widened if the zero-dispersion wavelength is aligned with a skirt of the OH absorption peak (1.38 xcexcm). In the case where the above-mentioned OH absorption peak is kept low by dehydration processing or the like, so as to utilize the wavelength band including the wavelength of 1.38 xcexcm as its signal light wavelength band, on the other hand, the zero-dispersion wavelength may be set within a wavelength range of longer than 1.45 xcexcm but not longer than 1.50 xcexcm in order to intentionally generate dispersion in the above-mentioned wavelength band.
In the optical fiber, while the dispersion slope monotonously increases, the absolute value of the dispersion slope at its zero-dispersion wavelength is 0.10 ps/nm2/km or less, and the dispersion slope at a wavelength of 1.55 xcexcm is preferably 0.06 ps/nm2/km or less, whereby the dispersion in the 1. 3-xcexcm wavelength band and the dispersion in the 1.55-xcexcm wavelength band are homogenized. Here, each of the absolute value of dispersion in the 1.3-xcexcm wavelength band and the absolute value of dispersion in the 1.55-xcexcm wavelength band is 6 ps/nm/km or more but 12 ps/nm/km or less.
As mentioned above, the optical fiber according to the present invention realizes efficient optical communications in both of the 1.3-xcexcm wavelength band and the 1.55-xcexcm wavelength band. From the viewpoint of guaranteeing a single mode, the case where the cutoff wavelength is 1.3 xcexcm or shorter while the transmission line length is several hundreds of meters or less is preferable since only the ground-mode light can propagate in each of the 1.3-xcexcm wavelength band and the 1.55-xcexcm wavelength band. Also, in view of the dependence of cutoff wavelength on distance, no practical problem occurs in optical transmission over a relatively long distance (a transmission line length of several kilometers or less) even if the cutoff wavelength is 1.45 xcexcm or shorter (in the case where it is longer than the signal light wavelength). From the viewpoint of reducing the bending loss, on the other hand, there are cases where the bending loss increases remarkably when the cutoff wavelength is shorter than 1.0 xcexcm. As a consequence, the cutoff wavelength is preferably 1.05 xcexcm or more, more preferably 1.30 xcexcm or more.
Further, in order to enable efficient optical transmission in the 1.3-xcexcm wavelength band and 1.55-xcexcm wavelength band, the optical fiber according to the present invention has a bending loss which becomes 0.5 dB or less, preferably 0.06 dB or less, per turn when wound at a diameter of 32 mm at a wavelength of 1.55 xcexcm, and has an effective area Aeff which becomes 45 xcexcm2 or more, preferably greater than 49 xcexcm2 at a wavelength of 1.55 xcexcm. Also, the amount of increase in transmission loss caused by OH absorption at a wavelength of 1.38 xcexcm in the optical fiber is 0.1 dB/km or less. In particular, if the amount of increase in transmission loss caused by OH absorption at a wavelength of 1.38 xcexcm is 0.1 dB/km or less, then a wavelength band in the vicinity of this wavelength of 1.38 xcexcm can be utilized for a signal light wavelength band. In this case, in order to intentionally generate dispersion in the wavelength band in the vicinity of the wavelength of 1.38 xcexcm (in order to suppress four-wave mixing), the zero-dispersion wavelength may be set within a wavelength range of longer than 1.45 xcexcm but not longer than 1.50 xcexcm.
Here, the effective area Aeff is given, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), by the following expression (1):                               A          eff                =                  2          ⁢                                                    π                ⁡                                  (                                                            ∫                      0                      ∞                                        ⁢                                                                  E                        2                                            ⁢                      r                      ⁢                                              ⅆ                        r                                                                              )                                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      ⅆ                    r                                                              )                                                          (        1        )            
where E is the electric field accompanying the propagated light, and r is the radial distance from the core center.
The optical fiber according to the present invention has a refractive index profile in which the maximum and minimum values of relative refractive index difference with reference to the refractive index of pure silica (silica which is not intentionally doped with impurities) are 1% or less and xe2x88x920.5% or more, respectively. In such a refractive index profile, the relative refractive index difference of a high refractive index region doped with Ge element, for example, with respect to pure silica is 1% or less, whereas the relative refractive index difference of a low refractive index region doped with F element, for example, with respect to pure silica is xe2x88x920.5% or more, whereby its manufacture (refractive index control by doping with impurities) is easy, and the transmission loss can be lowered. Here, the minimum value of relative refractive index difference with reference to the refractive index of pure silica is preferably xe2x88x920.2% or more, more preferably greater than xe2x88x920.15% from the viewpoint of facilitating the manufacture of the optical fiber.
The optical fiber having various characteristics such as those mentioned above can be realized by various configurations. Namely, a first configuration of the optical fiber can be realized by a structure comprising a core region which extends along a predetermined axis and has a predetermined refractive index, and a cladding region provided on the outer periphery of the core region. The optical fiber of the first configuration may further comprise a depressed cladding structure. The depressed cladding structure is realized when the above-mentioned cladding region is constituted by an inner cladding, provided on the outer periphery of the core region, having a lower refractive index than the core region; and an outer cladding, provided on the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding but lower than that of the core region.
As with the first configuration, a second configuration of the optical fiber comprises a core region and a cladding region provided on the outer periphery of the core region. However, the core region is constituted by a first core having a predetermined refractive index; and a second core, provided on the outer periphery of the first core, having a lower refractive index than the first core. In the case where the optical fiber of the second configuration comprises a depressed cladding structure, the cladding region is constituted by an inner cladding, in contact with the outer periphery of the second core, having a lower refractive index than the second core; and an outer cladding, provided on the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding but lower than that of the second core.
As with the first configuration, a third configuration of the optical fiber comprises a core region extending along a predetermined axis and a cladding region provided on the outer periphery of the core region. In particular, the core region comprises a first core having a predetermined refractive index; a second core, provided on the outer periphery of the first core, having a lower refractive index than the first core; and a third core, provided on the outer periphery of the second core, having a higher refractive index than the second core. In the case where the optical fiber of the third configuration comprises a depressed cladding structure, the cladding region is constituted by an inner cladding, in contact with the outer periphery of the third core, having a lower refractive index than the third core; and an outer cladding, provided on the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding but lower than that of the third core.
When the third configuration mentioned above is employed, it becomes easier to obtain an optical fiber having allow dispersion slope of 0.06 ps/nm2/km or less at a wavelength of 1.55 xcexcm in particular.
Further, a fourth configuration of the optical fiber also comprises a core region extending along a predetermined axis and a cladding region provided on the outer periphery of the core region. In particular, the core region comprises a first core having a predetermined refractive index; a second core, provided on the outer periphery of the first core, having a higher refractive index than the first core. In the case where the optical fiber of the fourth configuration comprises a depressed cladding structure, the cladding region is constituted by an inner cladding, in contact with the outer periphery of the second-rate, having a lower refractive index than the second core; and an outer cladding, provided on the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding but lower than that of the second core.
A fifth configuration of the optical fiber comprises a core region extending along a predetermined axis and a cladding region provided on the outer periphery of the core region. In particular, the core region comprises a first core having a predetermined refractive index; a second core, provided on the outer periphery of the first core, having a higher refractive index than the first core; a third core, provided on the outer periphery of the second core, having a lower refractive index than the second core; and a fourth core, provided on the outer periphery of the third core, having a higher refractive index than the third core. In this fifth mode of optical fiber, the cladding region has a lower refractive index than the fourth core.
The optical transmission system according to the present invention is realized by the optical fiber having such a configuration as those mentioned above. Specifically, the optical transmission system according to the present invention comprises, at least, a first transmitter for outputting first light in the 1.3-xcexcm wavelength band, a second transmitter for outputting second light in the 1.55-xcexcm wavelength band, a multiplexer for multiplexing the first light outputted from the first transmitter and the second light outputted from the second transmitter, and an optical fiber comprising a configuration mentioned above and having one end thereof optically connected to the multiplexer. As a result of this structure, the optical fiber transmits each of the first light and second light multiplexed by the multiplexer. According to the optical transmission system having such a structure, the first light in the 1.3-xcexcm wavelength band outputted from the first transmitter is made incident on the above-mentioned optical fiber by way of the multiplexer and propagates through the optical fiber toward a receiving system. On the other hand, the second light in the 1.55-xcexcm wavelength band outputted from the second transmitter is made incident on the optical fiber by way of the multiplexer and propagates through the optical fiber toward the receiving system. Also, as mentioned above, the optical fiber applied to the optical transmission line comprises a structure enabling efficient optical communications in each of the 1.3-xcexcm wavelength band and 1.55-xcexcm wavelength band, whereby the optical transmission system enables large-capacity communications when the optical fiber having such a special structure is employed therein.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.